DESCRIPTION (Applicant's Abstract): Cognitive functioning that can correctly process the structure in the environment is one of the hallmarks of mental health. Failing to mentally represent properties in the world accurately often indicates the presence of mental illness. Consequently, before psychologists can recognize when such failures occur they need to understand how normal cognitive processes operate. One important, perhaps fundamental, cognitive process is that of categorizing objects that are alike in some way into a common group, such as classifying an animate, winged object as a 'bird'. One reason to categorize an object as a kind of something is to distinguish the object from other objects that are not of the same kind (e.g., the object is a 'bird' and not a bat'), A different motivation would be to use information derived from knowledge of category membership to make predictions about the features of the object immediately at hand (e.g., if this is a bird then it is likely to sing and lay eggs). Underlying all of this is the need for perception of the attributes of an individual object so that this categorization, whatever the motivation, can take place. Researchers have assumed that this perception occurs prior to the categorization of the object and is mediated by a largely fixed set of features comprising the object. Recently, some researchers have questioned the independence of the perceptual process from the categorization process and the fixed-feature set hypothesis. That is, how we process attributes of an object, such as its size, color, shape, etc., may be influenced by the category into which we place it and what we know about that category. This proposal describes several experiments in which the structure of categories of simple visual objects is manipulated so that the effects on the low-level perception of the properties of those objects can be assessed. This proposal also contains experiments which assess how perceptual phenomena, akin to visual illusions, may alter or distort the memories of category properties. The goal is to establish the existence of these phenomena, and to accurately measure, in quantitative detail, their type and degree so that future models of this interactive process is possible.